The invention relates to a process for reclaiming used foundry sands, in which the used sands, if required, are freed from all adhering carbon-containing materials in a first step by a thermal treatment at 500.degree. to 900.degree. C. and subjected subsequently to a mechanical cleaning treatment. A process of the genus is already known from DE-OS 34 00 656.
In this known process, which is a continuous process, a reclaiming of used sands is carried out in three successive steps, namely a mechanical precleaning, a thermal treatment of the precleaned sand and a mechanical postcleaning. In this process the mechanical postcleaning is carried out after the carrying out of the thermal treatment in a rotating, cylindrical drum, in which baffles and/or grinding bodies such as balls or the like are provided.
This known mechanical postcleaning is detrimental inasmuch as the use of a rotating drum with baffles does not lead to a sufficient grain-to-grain friction to remove binding agent residues (bentonite) adhering to the sand grain. If the known mechanical cleaning process is carried out by means of grinding bodies a great portion of sand grains is destroyed. Since the known process takes place continuously, the known process cannot be adapted individually to different degrees of impurities of different used foundry sands. As a result it is not possible by means of the known process to claim used sands to new sand quality.
It is already known to accelerate used sands continuously by means of compressed air in a pipe and to hurl them against a baffle plate. Sand speeds of more than 20 m/s are used here, whereby up to 25% of the treated sand are smashed.
It is also already known to continuously subject used sands to a wet reclaiming, the used sand being treated with water so that carbon and clay particles are separated by the movement of the sand in the water. The disposal product is a moist sludge which contains up to 50% of hydrocarbons. The disposal of the washing water is problematical in this process for reasons of environmental protection and moreover the further use of the resultant disposal product is not ensured.
In this connection reference is made to the fact that the Waste Disposal Act having come into force in the Federal Republic of Germany in 1987 requires that used foundry sands are to be reclaimed and again supplied to the production process, because the former practice of depositing used sands on waste dumps will no longer be admissible in future.
The measures suggested so far for the reclaiming of used foundry sands failed in the last analysis because either the environment was inadmissibly burdened or because it was not possible to sufficiently remove the binding agent burnt onto the sand grain surfaces during the casting process (bentonite in the case of claybonded sands). As already mentioned, the hurling of used sand at high speed against stationary baffle surfaces, which is customary in technology, leads to considerable smashings of quartz sand of up to 25%, which leads to corresponding losses of sand.
Approx. 2.4 million tons of used foundry sands occur as waste products in waste dumps per year in the Federal Republic of Germany. This total amount of sand is divided into approx 30% of sand, which is found with artificial resin bonds and approx. 70% of sand which is found in clay-bonded fashion. The artificial-resinbonded sands are already partly reclaimed thermally, i.e. the sand is heated to approx. 800.degree. C., the organic, carbon-containing products adhering to the quartz grain being burnt. The clay-bonded sands with bentonite as binding agent are mainly obtained during the so-called wet casting mould systems.